On a typical August afternoon, in a typical American city, a case of the flu becomes the first salvo of a coordinated terrorist attack on the US homeland.

The Department of Health and Human Services and Centers for Disease Control under-react. The Transportation Safety Administration and Department of Transportation overreact. The Department of Homeland Security tries in vain to stay ahead of the crisis. It isn’t clear if the Department of Defense should or shouldn’t be involved. State and local authorities don’t know exactly what to do. And every agency at every level seems overwhelmed. By the time the worst has past, just over a fortnight after it begins, hospitals are pushed to the breaking point; foreign visitors are rounded up and quarantined; a high-rise office building is destroyed; dozens of people are killed; hundreds more are injured or infected; and a large swath of the country is thrown into chaos.

The good news is that none of this happened in the real world. Instead, it happened in a simulated, but all-too-realistic, virtual world created by the Homeland Security Institute at Purdue University (HSI). Dubbed “Measured Response 2004,” the exercise enables living, breathing decision makers to test their crisis-management skills in a computer-generated environment.

Measured Response brings together participants and planners representing an alphabet soup of federal and state agencies. (I was placed on the HHS team, along with a US Naval officer detached to the CDC and a state emergency management official.) Participants bring their own expertise and instincts to the war game, matching wits and wills against a terrorist Red Team.

The decisions and indecisions, actions and reactions, of participants are mixed together with countless other ingredients in what’s called the Synthetic Environment for Analysis and Simulation platform. SEAS relies on powerful computers to synthesize hundreds of independent factors (such as historical, economic and human-behavioral data), and approximate real-world results in real time. The platform then uses the computer equivalent of time-lapse photography to move events, actions, reactions and consequences along. According to HSI, “Over 500,000 artificial agents mimic the behavior of the citizens such as, mobility, the feeling of well being in terms of security (financial and physical), health, information, and civil liberties.”

Walls of high-resolution TVs give participants a bird’s eye view of what’s happening across the country and around the virtual world, while streaming email imitates the constant flow of ground-level information (and misinformation). Teams representing federal agencies are separated from those representing state and local agencies, which slows down communication, creates a fog of uncertainty and adds to the realism. Of course, as HSI Director Alok Chaturvedi observes, “Unlike the real world, we have a reset button.”

The exercise is made all the more real and relevant by the latest round of homeland threats on New York and Washington, DC, and by a cursory glance at the roster of nations armed with biological weapons:

GlobalSecurity.org reports that “Iran’s biological warfare program is now believed to generally be in the advanced research and development phase. Iran has qualified, highly trained scientists and considerable expertise with pharmaceuticals. It also possesses the commercial and military infrastructure needed to produce basic biological warfare agents.” Syria has “a robust biotechnology infrastructure…It is highly probable that Syria also is developing an offensive biological warfare capability.”

In 2001, the United States alleged that North Korea had “a dedicated, national-level effort to achieve a BW capability and that it has developed and produced, and may have weaponized, BW agents in violation of the [Biological Weapons] Convention.” Syria and Iran are also signatories to the treaty banning biologicals, but why would that stop them from breaching it? As Hitler said of an earlier treaty, it’s only a “scrap of paper.”

Despite Muammar Qaddafi's stunning pre-emptive surrender at the end of 2003, Libya represents a cautionary tale of the spread of weapons of mass destruction. Qadaffi was knee-deep in biological and chemical weapons programs before he saw a mangy Saddam Hussein cowering in a hole—and perhaps caught a glimpse of his own fate.

Then there are the transnational threats, whose foot soldiers and leaders are not impressed by such images or deterred by armies.

But interdiction and proliferation are beyond the scope of Measured Response. According to HSI, the objective of Measured Response is simply “to develop and analyze policies and operating procedures to manage the public mood, maintain public health, mitigate the risk of contagion, maintain orderly movement of traffic and people, and apprehend perpetrators.” Along the way, participants gain a better understanding of how they will make decisions in a crisis situation. Another important goal, according Chaturvedi, is “to make participants think more imaginatively.”

By that measure, the exercise was a success.

But where there is good news, there is usually bad news. The bad news is that what transpired in HSI’s virtual world, with its reset buttons, two-dimensional buildings and bloodless victims, can happen in our world.

The worse news is the unspoken premise under which Measured Response and similar exercises like 2001’s Dark Winter operate—the notion that such an attack is so likely, so inevitable, that we have to prepare for it rather than prevent it. It’s called “Measured Response” for good reason—the participants are responding to an attack.

This is not a criticism of the exercise or its planners. If anything heartening can come from witnessing the fast-motion carnage and chaos of Measured Response, it’s the reassuring sense that some of America’s best and brightest are developing systems to shorten reaction times, bracing America’s emergency infrastructure for the next attack, and grappling with nightmares that the rest of us would rather not contemplate on a typical afternoon in August.